Method for forming resist patterns having two photoresist layers and an intermediate layer

ABSTRACT

A method for forming resist patterns having two photoresist layers and an intermediate layer involves coating a primary photoresist film having a small thickness over an under layer, and exposing the primary photoresist film to light using a mask. Then, the primary photoresist film is developed, thereby forming a primary photoresist film pattern. An intermediate layer is formed over the entire exposed surface of a resulting structure, the intermediate layer being made of a spin on glass or plasma enhancement oxide. A secondary photoresist film is coated over the intermediate layer. The secondary photoresist film is exposed to light using the same mask as used for the primary photoresist film, and is developed to form a secondary photoresist pattern. A portion of the intermediate layer exposed through the secondary photoresist pattern is etched, forming a secondary photoresist pattern completely overlapping with the primary photoresist pattern so that the resulting resist pattern has a vertical profile.

This is a continuation of application Ser. No. 08/501,947, filed Jul.13, 1995, now U.S. Pat. No. 5,830,624, issued Nov. 3, 1998.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a method for forming resist patterns inthe fabrication of semiconductor devices, and more particularly, to amethod for forming resist patterns having a vertical profile.

2. Description of the Prior Art

Where a resist pattern is formed using conventional single layer resistand tri-level resist methods, a notching occurs at side walls of theresist pattern due to light reflected by a substrate disposed under theresist pattern or an under layer. Such a notching results in a problemthat the resist pattern has no vertical profile.

FIG. 1 illustrates the exposure of a resist film to light in accordancewith a conventional method. As shown in FIG. 1, a photoresist film 34 iscoated over a substrate or an under layer 35. As the photoresist film 34is exposed to light 31 through a mask 32, the light transmitting thephotoresist film 34 is reflected from the surface of the substrate 35and then reflected again from the upper surface of the photoresist film34. As a result, non-exposure portions of the photoresist film 34 areundesirably exposed to light 36 irregularly reflected as mentionedabove. This results in a notching phenomenon that the side walls of aphotoresist film pattern are partially removed.

Where a resist pattern is formed by coating a photoresist film over asubstrate formed with another pattern and then patterning thephotoresist film by use of exposure and development processes, anotching may occur due to the topology of the under pattern. In otherwords, the photoresist film pattern is partially removed at its sidewalls.

Generally, photoresist films used to obtain micropatterns are resistfilms having a low viscosity and a high sensitivity. Due to theseproperties, it is difficult for such photoresist films to have a largethickness. It is impossible to use photoresist films having a smallthickness as masks for patterning metal layers. In order to increase thephotoresist film thickness, a double resist coating method has beenproposed.

FIG. 6 illustrates a conventional double resist coating method. Inaccordance with this method, a primary photoresist film 43 is formedover a silicon substrate 44 by coating a liquid phase photoresistmaterial 46 through a nozzle 41 onto the silicon substrate 44 mounted onthe shaft 45 of a spinner while rotating the silicon substrate 44, asshown in FIG. 6. A secondary photoresist film 42 is coated cover theprimary photoresist film 43.

During the coating of the secondary photoresist film 42, the uppersurface of the primary photoresist film 43 is dissolved by a solventcontained in the material for the secondary photoresist film 42, therebycausing an interface between the primary photoresist film 43 and thesecondary photoresist film 42 to be poor. As a result, the primary andsecondary photoresist films have a degraded uniformity in thickness. Asthe photoresist film thickness becomes larger, the resolution of theresulting resist pattern is degraded. This is because the undesirablylarge thickness of the photoresist film results in limiting the depth offocus which causes the process margin of the reduced exposure equipmentto decline.

SUMMARY OF THE INVENTION

Therefore, an object of the invention is to solve the problemencountered in the conventional method illustrated in FIG. 1 and toprovide a method for forming a resist pattern having a vertical profileby absorbing light reflected from the surface of an under substrate toform a thin primary photoresist pattern.

Another object of the invention is to solve the problem encountered inthe conventional method illustrated in FIG. 6 and to provide a methodfor forming a resist pattern having a good profile by coating a primaryphotoresist film, exposing the primary photoresist film to light,coating an anti-reflective layer over the primary photoresist film,coating a secondary photoresist film over the anti-reflective layer,exposing the secondary photoresist film to light, and developing theresulting structure to form primary and secondary resist patterns.

In accordance with an aspect, the present invention provides a methodfor forming a resist pattern, comprising the steps of: coating a primaryphotoresist film having a small thickness over an under layer; exposingthe primary photoresist film to light using a mask and then developingthe primary photoresist film, thereby forming a primary photoresistpattern; forming an intermediate layer over the entire exposed surfaceof the resulting structure; coating a secondary photoresist film overthe intermediate layer, exposing the secondary photoresist film to lightusing the same mask as used for the primary photoresist film and thendeveloping the secondary photoresist film, thereby forming a secondaryphotoresist pattern; and etching a portion of the intermediate layerexposed through the secondary photoresist pattern, thereby forming asecondary photoresist pattern completely overlapping with the primaryphotoresist pattern so that the resulting resist pattern has a verticalprofile.

In accordance with another aspect, the present invention provides amethod for forming a resist pattern, comprising the steps of: coating aprimary photoresist film over an under layer, exposing the primaryphotoresist film to light using a mask, thereby defining a light-exposedregion in the primary photoresist film; forming an anti-reflective layerhaving a small thickness over the primary photoresist film; coating asecondary photoresist film over the anti-reflective layer, and exposingthe secondary photoresist film to light using the same mask as used forthe primary photoresist film, thereby defining a light-exposed region inthe secondary photoresist film; and developing the resulting structure,thereby removing portions of the primary and secondary photoresist filmsand anti-reflective layer respectively corresponding to thelight-exposed region so that the resulting resist pattern has a verticalprofile.

BRIEF DESCRIPTION OF THE DRAWINGS

Other objects and aspects of the invention will become apparent from thefollowing description of embodiments with reference to the accompanyingdrawings in which:

FIG. 1 is a sectional view illustrating the exposure of a resist film tolight in accordance with a conventional method;

FIGS. 2 to 5 are sectional views respectively illustrating thesequential steps involved in a method for forming a resist pattern inaccordance with a first embodiment of the present invention;

FIG. 6 is a sectional view illustrating a conventional double resistcoating method;

FIGS. 7 to 10 are sectional views respectively illustrating thesequential steps involved in a method for forming a resist pattern inaccordance with a second embodiment of the present invention; and

FIG. 11 is a sectional view-illustrating a multi-layer resist structureformed in accordance with a third embodiment of the present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

FIGS. 2 to 5 illustrate the sequential steps involved in a method forforming a resist pattern in accordance with a first embodiment of thepresent invention, respectively.

In accordance with this method, a thin primary photoresist film 7 iscoated over a substrate 5, as shown in FIG. 2. The primary photoresistfilm 7 is then exposed to light using a mask 2, thereby forming alight-exposed region 8.

The primary photoresist film 7 has a small thickness of not more than1,000 Å to obtain an improvement in the depth of focus. Accordingly, itis possible to obtain an improvement in the resolution of a resistpattern even when small exposure energy is used. However, it isdifficult to use the primary photoresist film 7 having such a smallthickness as a satisfactory etch mask for the step of etching an underlayer disposed beneath the primary photoresist film.

Thereafter, the primary photoresist film 7 is partially removed at itsportion corresponding to the light-exposed region 8 using a developmentprocess, thereby forming a primary photoresist pattern 7', as shown inFIG. 3. A hard baking is then carried out at a temperature ranging from150° C. to 300° C. to cure the primary photoresist pattern 7'. Over theresulting structure, an intermediate layer 10 is deposited to a smallthickness ranging, for example, from 500 Å to 2,000 Å. The intermediatelayer 10 is adapted to prevent the primary photoresist pattern 7' frombeing damaged upon coating a secondary photoresist film duringsubsequent steps. The intermediate layer 10 is made of a spin on glass(SOG) or a plasma enhanced oxide. The intermediate layer 10 may also bemade of anti-reflective coating material which is not any metallicmaterial, but a material having a high transparency such as polyvinylalcohol. A secondary photoresist film 9 is then coated over theintermediate layer 10. Using the mask 2 used at the step of FIG. 2, thesecondary photoresist film 9 is then exposed to light 1.

With such a structure, the effect of reflective light 11 resulted fromthe exposure light is minimized to achieve an improvement in contrast.In other words, the light transmitting the secondary photoresist film 9is reflected from the surface of the substrate 10 and then reflectedagain from the surface of the secondary photoresist film 9. Thereflected light is then absorbed into the primary photoresist pattern7', thereby minimizing the problem that the secondary photoresist film 9is exposed at its non-exposure portion to the reflected light.

The thickness of the secondary photoresist film 9 is determined by thethickness of the under layer to be patterned, namely, the layer disposedbeneath the secondary photoresist film 9.

FIG. 4 shows a light-exposed region 3 defined in the secondaryphotoresist film 9 during the light exposure step.

The resulting structure is then developed to remove the light-exposedportion 3 of the secondary photoresist film 9, thereby forming asecondary photoresist-pattern 9', as shown in FIG. 5. Thereafter, theintermediate layer 10 is partially etched at its portion exposed throughthe secondary photoresist pattern 9'. As a result, a resist patternhaving a vertical profile is formed.

On the other hand, FIGS. 7 to 10 illustrate the sequential stepsinvolved in a method for forming a resist pattern in accordance with asecond embodiment of the present invention, respectively.

In accordance with this method, a primary photoresist film 13 is coatedto a thickness of not more than 10,000 Å over a silicon substrate 14, asshown in FIG. 7. The primary photoresist film 13 is then exposed tolight using a mask 12, thereby forming a light-exposed region 20.

Over the resulting structure, an anti-reflective layer is formed to athickness of, for example, several hundreds angstroms to several tenhundreds angstroms to protect the primary photoresist film 13 and thelight-exposed region 20, as shown in FIG. 8. The anti-reflective layer15 is made of polyimide or other anti-reflective coating (ARC) materialinsoluble in a solvent contained in the material of the primaryphotoresist film 13 and exhibiting a high light absorption. The ARC issoluble in H₂ O contained in a developer.

A secondary photoresist film 16 is then lightly coated over thenon-reflective layer 15, as shown in FIG. 9. Using the mask 18 used atthe step of FIG. 7, the secondary photoresist film 16 is then exposed tolight 17, thereby defining a light-exposed region 21.

The resulting structure is then developed to remove the light-exposedportion 21 of the secondary photoresist film 16, the portions of theanti-reflective layer 15 and primary photoresist film 13 respectivelydisposed beneath the light-exposed portion 21. As a result, a secondaryphotoresist pattern 16', a non-reflective layer pattern 15' and aprimary photoresist pattern 13' are formed.

After the formation of the patterns, the anti-reflective layer 15 isremoved using a developer used at the development step.

The method of the second embodiment involves coating the primaryphotoresist film having a small thickness, exposing the primaryphotoresist film to light, coating the secondary photoresist film overthe patterned primary photoresist film and exposing the secondaryphotoresist film to light on the basis of the fact that a thin resistfilm can provide a sufficient process margin for the depth of focus uponbeing exposed to light. Therefore, this method can obtain a good profileof the resist pattern in spite of the fact that the resist pattern has alarge thickness.

FIG. 11 illustrates a multi-layer resist structure formed in accordancewith a third embodiment of the present invention.

AS shown in FIG. 11, the multi-layer resist structure includes a primaryphotoresist film 55, a primary anti-reflective film 56, a secondaryphotoresist film 57, a secondary anti-reflective film 58 and a thirdphotoresist film 59 sequentially formed over a substrate 54. Themulti-layer resist film has a thickness of 7 to 8 μm. The photoresistfilms are exposed to light using the same mask in a manner as describedin conjunction with the second embodiment. Accordingly, the resultingresist pattern has a vertical profile.

As apparent from the above description, the first embodiment of thepresent invention involves forming an intermediate layer over a primaryphotoresist pattern and coating a secondary photoresist film over theintermediate layer. The primary photoresist pattern absorbs lightirregularly reflected upon exposing the secondary photoresist film tolight, thereby minimizing a notching phenomenon occurring at a patternof the secondary photoresist film.

Also, the second embodiment of the present invention involves a doubleresist coating, namely, coating of primary and secondary photoresistfilms. As the thickness of the resulting resist structure is increasedby virtue of the double resist coating, it is possible to form a resistpattern having a good profile.

Although the preferred embodiments of the invention have been disclosedfor illustrative purposes, those skilled in the art will appreciate thatvarious modifications, additions and substitutions are possible, withoutdeparting from the scope and spirit of the invention as disclosed in theaccompanying claims.

What is claimed is:
 1. A method for forming a resist pattern,comprising:coating a primary photoresist film over an under layer to athickness of not more than 1000 Å so as to obtain an improvement in adepth of focus; exposing the primary photoresist film to light using amask, and then developing the primary photoresist film, and removing aportion of the primary photoresist film corresponding to a light-exposedregion to form a primary photoresist pattern; forming an intermediatelayer made of spin on glass or a plasma enhanced oxide and having athickness of 500 to 2000 Å over the entire exposed surface of theresulting structure which includes the primary photoresist pattern toprevent the primary photoresist pattern from being damaged upon coatinga secondary photoresist film; coating the secondary photoresist filmover the intermediate layer and exposing the secondary photoresist filmto light using the same mask as used for the primary photoresist filmand then developing the secondary photoresist film, and removing aportion of the secondary photoresist film corresponding to thelight-exposed region to form a second photoresist pattern; and etching aportion of the intermediate layer exposed through the secondaryphotoresist pattern using the secondary photoresist pattern as a mask tocompletely overlap the secondary photoresist pattern with the primaryphotoresist pattern so that the resulting resist pattern has a verticalprofile.
 2. A method in accordance with claim 1, furthercomprising:baking the primary photoresist pattern at a temperatureranging from 150° C. to 300° C. after the step of forming the primaryphotoresist pattern.